Multi-fibre arrangement for high power fibre lasers and amplifiers

ABSTRACT

An optical amplifier includes at least one pump source and an optical fiber cable which includes an amplifying optical fiber and a pump optical fiber that are defined by respective lengths. The amplifying optical fiber and the pump optical fiber are coated with a common coating along a portion of their respective lengths, and the fibers are in optical contact with each other along a coating length within the common coating. The common coating has a refractive index which is lower than a refractive index of a cladding material of the pump optical fiber. The fibers are made substantially from glass. The amplifying optical fiber includes a core and a cladding, and is doped with a rare earth dopant. The pump optical fiber is defined by a first end and a second end, the first end of the pump optical fiber being connected to the pump source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. §120 to U.S. patent application Ser. No. 12/658,610, filed Feb.8, 2010 (now U.S. Pat. No. 8,270,070), which is a continuation of, andclaims priority under 35 U.S.C. §120 to U.S. patent application Ser. No.11/788,054, filed Apr. 19, 2007 (now U.S. Pat. No. 7,660,034), which isa continuation of, and claims priority under 35 U.S.C. §120 to U.S.patent application Ser. No. 10/999,758, filed Nov. 29, 2004 (now U.S.Pat. No. 7,221,822), which is a continuation of, and claims priorityunder 35 U.S.C. §120 to U.S. patent application Ser. No. 09/560,593,filed Apr. 28, 2000 (now U.S. Pat. No. 6,826,335), which in turn claimspriority under 35 U.S.C. §119 to the following U.K. patent applications:serial number 9910165.1, filed in the United Kingdom on Apr. 30, 1999;serial number 9911958.8, filed in the United Kingdom on May 21, 1999,which is a continuation-in-part of, and claims priority to, UnitedKingdom patent application serial number 9910165.1 (identified above);and serial number 9917594.5, filed in the United Kingdom on Jul. 27,1999. All of the foregoing applications are incorporated herein byreference in their entirety where appropriate.

FIELD OF THE INVENTION

This invention relates to an optical fibre arrangement, optical fibrelasers and optical fibre amplifiers.

BACKGROUND OF THE INVENTION

There is a demand for optical amplifiers that can output powers of 1 Wor greater, can amplify many wavelength channels simultaneously with lowcross-talk and low inter-channel interference, and can do so with highreliability and low cost per wavelength channel.

In many applications such as dense wavelength division multiplexing(WDM) transmission systems and satellite communications, opticalamplifiers and transmitters, optically pumped by, e.g., laser diodes,should not only be capable of handling relatively high power but also beprotected against failure of pump sources.

Conventional optical amplifiers use single-mode optical fibre whose coreis doped with one or more rare-earth ions such as Erbium. Suchamplifiers provide limited power output that is insufficient formulti-channel WDM transmission systems. In addition, conventionalamplifiers are prone to the failure of pump sources, requiring severalpump sources to be contained with the amplifier in order to provide pumpredundancy—but at high cost.

The power output of conventional optical amplifiers has recently beenincreased by the introduction of pump modules containing severalsemiconductor lasers whose outputs are wavelength division multiplexedinto a single optical fibre. Although the output power obtainable fromsuch an optical amplifier containing one of these pump modules issufficient for amplifying many channels simultaneously, the approach isexpensive, is currently limited in powers to around 1 W, and offerslimited pump redundancy.

Higher-power optical amplifiers and fibre lasers can be constructedusing double-clad optical fibres containing a single-mode waveguidingcore doped with rare-earth ions (such as Erbium or Erbium/Ytterbium) anda multi-mode inner cladding formed by the silica cladding guidingagainst an outer cladding with lower refractive index. This outercladding is typically a polymer outer cladding. However, it iscumbersome to separate the signal path to the single-mode core from thepath required to launch pump powers into the inner cladding. Severaltechniques have been tried including separating the beams with dichroicmirrors, side pumping using a multimode coupler, and etching pits intothe double-clad fibre. However, none of these techniques provides asimple, effective way of reliably introducing the pump energy into theoptical amplifier or fibre laser—especially if several pump lasers arerequired in order to provide pump redundancy. This issue is of concernfor high-power fibre lasers where there is a requirement to introducethe pump energy from several to tens of pump diodes into the lasercavity. No effective, reliable and cost-effective way to achieve thisexists in the prior art.

An associated problem is that introducing signal conditioning into theoptical amplifier can be difficult. For example, it is often desirableto compensate for the spectral gain variation within the opticalamplifier, or to introduce a filter to compensate for the dispersion ina telecommunication link. This requires ready access to the signal,which can be difficult for most amplifier configurations. A requirementtherefore exists for an amplifier and laser design where it is simple toinsert added functionality.

Today's optical telecommunications networks are increasingly based onwavelength division multiplexing—the simultaneous transmission of manywavelength channels through the same fibre. As the networks expand,these wavelength channels can originate from different locations. Thisplaces stringent demands on the management of the network, especially onthe performance of optical amplifiers dispersed throughout the network.The wavelength channels arriving at an optical amplifier are unlikely tohave equal powers (i.e., they are unbalanced), and the power of anindividual wavelength channel can be suddenly and unexpectedlyincreased. This unbalance and the changing of the power levels inindividual channels is referred to as granularity. Prior art opticalamplifiers experience problems with unbalanced wavelength channels inthat the highest power wavelength channel can be amplified more than theother channels, thus increasing the unbalance. In addition, the suddenchanging of the power level in one wavelength channel can causeinstabilities in the optical amplifier that carry over to otherchannels. One of the most robust solutions to remove the granularity isto separate all the wavelength channels prior to amplification, amplifythe channels, and then recombine the channels for retransmission. Themajor problem with this approach is that networks can transmit over onehundred wavelength channels through a single optical fibre. The cost ofprior-art optical amplifiers makes this solution unattractive.

The cost issue of optical amplifiers is also a problem as the networksexpand into metropolitan areas, the expansion being driven by theinsatiable demand for bandwidth for internet, data, mobile phones andcable television. Prior art optical amplifiers are too expensive andthis is currently limiting the expansion of the networks.

Erbium-doped fibre amplifiers have revolutionized opticaltelecommunications over the last ten years. They are finding more andmore uses, for instance for compensation of switching losses. Theincreasing need for capacity in telecommunication networks drives notonly amplification requirements, e.g., output power and gain flatnessfor wavelength division multiplexing applications, but also the requirednumber of amplifiers in a system. Erbium doped fibre amplifiers haveremained “stand-alone” devices, with individual amplifiers separatelypackaged. Component count as well as cost then holds back penetration ofthe optical amplifiers into different application areas that require alarge number of amplifiers at a low cost. Instead, the drive has beentowards purpose-built optical amplifiers with high specifications(bandwidth and output power) for use in applications that can tolerate ahigh cost.

It is therefore an aim of the present invention to obviate or reduce theabove mentioned problems.

SUMMARY OF THE INVENTION

According to a non-limiting embodiment of the present invention, thereis provided an optical fibre arrangement comprising at least two opticalfibre sections, the optical fibre sections each having an outsidelongitudinally extending surface, and the outside longitudinallyextending surfaces being in optical contact with each other.

The invention further includes an optical amplifier constructed fromsuch an optical fibre arrangement, and especially a parallel opticalamplifier with multiple amplifying fibres. This embodiment of theinvention realizes particular commercial application in opticaltelecommunication networks.

The apparatus and methods of the invention can enable pump power to beconveniently coupled into optical amplifiers and lasers.

The apparatus and methods of the invention can enable optical amplifiersand lasers to be constructed that are more immune to pump failure thanare prior art devices.

The apparatus and methods of the invention can enable optical amplifiersand lasers to be conveniently constructed having additionalfunctionality.

The apparatus and methods of the invention can enable a route for lowercost optical amplification particularly useful in optical networks.

The apparatus and methods of the invention can reduce the effects ofgranularity in optical networks.

The apparatus and methods of the invention can enable individualwavelength channels in WDM networks to be amplified and balanced.

The apparatus and methods of the invention can enable high-power opticalamplifiers and high-power fibre lasers to be constructed.

The invention also provides an optical fibre arrangement comprising aplurality of optical fibres each having an outside surface and defininga length, and wherein the outside surface of at least two adjacentoptical fibres are in optical contact along at least a respectiveportion of their lengths.

The optical fibre arrangement can comprise a plurality of optical fibresthat are surrounded by a coating material along the length of theoptical fibre arrangement.

The invention also provides a method for manufacturing an optical fibrearrangement comprising the following steps: providing a plurality ofoptical fibre preforms, each optical fibre preform comprising aplurality of optical fibres, each optical fibre defining an outsidesurface and a length, mounting the plurality of optical fibre preformsin a fibre drawing tower, drawing a plurality of optical fibre from theplurality of optical fibre preforms under a drawing tension and at adrawing speed, the drawing tension and the drawing speed being selectedsuch that the outside surface of at least two adjacent optical fibresare in optical contact along at least a respective portion of theirlengths. The plurality of optical fibres can be twisted or intertwinedduring the drawing process.

The plurality of optical fibres can be coated by passing the fibresthrough a coating cup filled with a coating material.

The invention also provides a method for manufacturing an optical fibrearrangement comprising the following steps: providing a plurality ofoptical fibres, each optical fibre defining an outside surface and alength, pulling the plurality of optical fibre under a drawing tensionand at a drawing speed, the drawing tension and the drawing speed beingselected such that the outside surface of at least two adjacent opticalfibres are in optical contact along at least a respective portion oftheir lengths. The plurality of optical fibres can be twisted during thedrawing process.

The invention also provides an amplifying optical device having anoptical pump and an optical fibre arrangement comprising a plurality oflengths of at least one optical fibre, each length of the optical fibredefining a longitudinally extending outside surface, the arrangementbeing such that the outside surfaces of at least two adjacent lengths ofthe optical fibre are in optical contact with each other.

The amplifying optical device can be an amplifier comprising a pluralityof amplifying fibres, each having an input and an output, at least onepump optical fibre having two ends, and a pump that supplies pump energyconnected to the pump optical fibre, the amplifier being configured suchthat the pump energy is shared by the plurality of amplifying fibres.

The amplifying optical device can be an amplifier comprising at leastone input fibre, a first multiplexer connected to the input fibre, acoupler, and at least one output port connected to the coupler, theamplifier being configured such that at least one of the amplifyingoptical fibres is connected to the first multiplexer and at least one ofthe amplifying optical fibres is connected to the coupler.

The fiber arrangement can serve to couple light from one or more pumpfibers into one or more signal fibers. The signal fibers can incorporatea core for guiding signal (or generated in case of a laser) lightthrough the arrangement. The cores can be single-moded. The regionsurrounding the cores of these fibers is capable of guiding pump light.These signal fibers each have two ends, at least one of which isaccessible, for example in the sense that other fibers can be spliced tosaid signal fibres. The pump fibers are capable of guiding highlymulti-moded pump beams from a pump source into the arrangement. The pumpfibers each have two ends. Light can be launched into the pump fibersthrough their ends.

The invention also provides an amplifying arrangement comprising aplurality of optical amplifiers each having a plurality of amplifyingoptical fibres and further comprising a second multiplexer connected toeach first multiplexer.

The amplifying optical arrangement can comprise a plurality of opticalamplifiers and an optical device, the amplifying optical arrangementbeing configured such that the optical device is connected to at leastone optical amplifier.

The optical device can comprise at least one of an optical router, anoptical switch, a gain flattening filter, a polarizer, an isolator, acirculator, a grating, an optical fibre Bragg grating, a long-periodgrating, an acousto-optic modulator, an acousto-optic tunable filter, anoptical filter, a Kerr cell, a Pockels cell, a dispersive element, anon-linear dispersive element, an optical switch, a phase modulator, aLithium Niobate modulator, or an optical crystal.

The invention also provides an amplifying optical device comprising afibre arrangement formed as a coil of a plurality of turns of amplifyingoptical fibre, the fibre comprising an inner core and an outer cladding,the arrangement being such that the claddings of adjacent fibres of atleast a pair of the turns touch one another. The coil can be coated, andthe coil can comprise at least one amplifying optical fibre and at leastone pump optical fibre.

An amplifying optical device such as a laser or an optical amplifierconstructed from a coil of uncoated optical fibre has the followingadvantages compared to the prior art:

-   -   1. It is based on an all-glass amplifying fibre: This overcomes        the power limitations associated with polymer outer claddings.    -   2. It has a single pump-guiding cladding with an embedded core,        but no coating or outer cladding. This overcomes the problems        with accessing the pump waveguide for side-splicing that arises        in other all-glass structures.    -   3. It uses a glass-air waveguide for guiding the pump. This        results in a high NA>1.    -   4. It has a substantially reduced glass-air surface area        compared to previous fibre laser designs. This reduces the        losses that arise at such an interface.    -   5. It can be formed by coiling a fibre and fixing it (e.g., by        fusing) into a rigid body.    -   6. It eliminates the requirement (in a prior-art cladding-pumped        fibre amplifier or fibre laser) that the fibre from which this        new structure is made must be able to guide the pump i.e., is        large enough and with sufficient NA (and assuming that the        amplifier or laser structure is made by coiling a fibre). Thus,        a fibre with a much smaller outer diameter and hence a much        lower passive-cladding to active-core volume ratio (=area ratio)        can be used. This improves pump absorption and thereby        efficiency. Instead, it is enough that the structure as a whole        can guide the pump. For instance, a pump coupler can be        side-spliced to the structure rather than to a single point on a        single fibre or the pump energy can be introduced with a        plurality of pump optical fibres that can be of such a size that        they can be located in the interstitial spaces within the coil.    -   7. It eliminates the need for special geometries for improved        pump absorption. In a prior-art cladding-pumped fibre, special        measures like off-center cores, non-circular claddings, or        bending of the fibre to special geometries is normally needed to        improve the pump absorption because some pump modes are        otherwise absorbed too slowly. The disclosed structure can help        to eliminate the geometrical similarities and symmetries between        the pump waveguide and signal waveguide (core). This improves        the pump absorption even in the absence of any further measures        as described above.    -   8. It can be securely supported at a few points in space. This        reduces any excess pump propagation loss that can arise at such        supporting points, because these perturb the waveguiding        air-glass interface. The structure can be supported by the fiber        pig-tails. These are typically coated, and so can be fixed        without any pump loss.    -   9. It provides means for preventing pump light from leaking out        through pump delivery fibres.

The invention also provides an optical fibre laser comprising anamplifying optical device comprising a pump source and an optical fibrearrangement, and an optical feedback arrangement for promoting lightgeneration within the laser.

The invention also provides a method for reducing the granularity inoptical telecommunications network, which method comprises providing atleast one amplifying optical arrangements as described herein having aplurality of amplifying optical fibres in at least one location withinthe network.

The invention also provides an optical telecommunications networkcomprising at least one of the amplifying optical arrangements describedherein, and having a plurality of amplifying optical fibres.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described solely by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 is a diagram of a prior art fibre amplifier;

FIG. 2 is a diagram of a prior art double clad fibre structure;

FIG. 3 is a diagram of a prior art fibre laser;

FIG. 4 is a diagram of a prior art pump scheme;

FIG. 5 is a diagram of a prior art multiple pump scheme;

FIG. 6 is a diagram of a prior art pump coupling scheme;

FIG. 7 is a diagram of an embodiment of the present invention;

FIG. 8 is a diagram of an embodiment of the present invention in whichthe optical fibre sections are from the same optical fibre;

FIG. 9 is a diagram of an embodiment of the present invention in whichthe optical fibre sections are from different optical fibres;

FIG. 10 is a diagram of an embodiment of the present invention where thefibres have different diameters;

FIG. 11 is a diagram of an embodiment of the present invention in whichthe optical fibre sections are fused together;

FIGS. 12 to 19 depict various optical fibre arrangements with aplurality of first and second optical fibres according to the presentinvention;

FIG. 20 depicts an optical fibre arrangement according to the presentinvention including a holey fibre;

FIG. 21 depicts an optical fibre arrangement according to the presentinvention in which the optical fibre sections are twisted with respectto each other;

FIG. 22 depicts an optical fibre arrangement according to the presentinvention in which a pump optical fibre is twisted around an amplifyingoptical fibre;

FIG. 23 depicts an optical fibre arrangement according to the presentinvention in which a pump optical fibre is twisted around two amplifyingoptical fibres;

FIG. 24 depicts an optical fibre arrangement according to the presentinvention in which six second optical fibres are twisted about a firstoptical fibre;

FIG. 25 depicts an optical fibre arrangement according to the presentinvention in which third optical fibres are twisted around first andsecond optical fibres;

FIG. 26 depicts an optical fibre arrangement according to the presentinvention in which the optical fibre sections are coated;

FIG. 27 depicts an apparatus for manufacturing an optical fibrearrangement according to the present invention;

FIG. 28 depicts an apparatus for manufacturing an optical fibrearrangement having a coating according to the present invention;

FIG. 29 depicts an optical fibre arrangement according to the presentinvention in which the optical fibre sections are held together with anoptical glue;

FIG. 30 depicts an amplifying optical device according to the presentinvention;

FIG. 31 depicts an amplifying optical device according to the presentinvention in which a pump optical fibre is twisted around an amplifyingoptical fibre;

FIG. 32 depicts an amplifying optical device according to the presentinvention comprising two pump optical fibres;

FIG. 33 depicts an amplifying optical device according to the presentinvention comprising two amplifying optical fibres;

FIGS. 34 to 36 depict an amplifying optical device according to thepresent invention in which the amplifying optical fibres are joinedtogether in different ways;

FIG. 37 depicts an amplifying optical device according to the presentinvention and including an optical element;

FIG. 38 depicts an amplifying optical device according to the presentinvention in which the optical element is an optical fibre Bragggrating;

FIG. 39 depicts an amplifying optical device according to the presentinvention in which the optical element connects two amplifying opticalfibres;

FIG. 40 depicts an amplifying optical device according to the presentinvention in which an optical element and a reflecting device areconfigured to reflect optical energy to the same amplifying opticalfibre;

FIG. 41 depicts an amplifying optical device according to the presentinvention in which an amplifying optical fibre is configured in a coil;

FIG. 42 depicts an amplifying optical device according to the presentinvention in which an amplifying optical fibre is configured in a coiland including a pump optical fibre;

FIG. 43 depicts an amplifying optical device according to the presentinvention in which an amplifying optical fibre and multiple pump opticalfibres are configured in a coil;

FIG. 44 depicts an amplifying optical device according to the presentinvention in which multiple amplifying optical fibres are configured ina coil;

FIG. 45 depicts an amplifying optical device according to the presentinvention in which at least one pump optical fibre is disposed ininterstitial gaps between turns of at least one amplifying opticalfibre;

FIG. 46 depicts an amplifying optical device according to the presentinvention in which an amplifying optical fibre is wound around a former;

FIG. 47 depicts an amplifying optical device according to the presentinvention in which an amplifying optical fibre and a pump optical fibreare wound around a former;

FIG. 48 depicts a laser according to the present invention;

FIG. 49 depicts a laser according to the present invention configured asa ring laser;

FIG. 50 depicts an optical amplifier according to the present invention;

FIG. 51 depicts an amplifier according to the present inventioncomprising a first multiplexer;

FIG. 52 depicts an amplifying optical arrangement according to thepresent invention;

FIG. 53 depicts an amplifying optical arrangement according to thepresent invention comprising an optical device;

FIG. 54 depicts an optical network according to the present invention;

FIG. 55 depicts a power splitter according to the present invention;

FIG. 56 depicts a serial power splitter according to the presentinvention;

FIG. 57 depicts a power splitter and amplifiers according to the presentinvention;

FIG. 58 depicts an amplifier according to the present invention; and

FIGS. 59 to 62 depict performance results measured on an amplifieraccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a schematic diagram of a conventional optical amplifier 10according to prior art. The optical amplifier 10 is based on an erbium(Er)-doped optical fibre 11 that is optically pumped by two pump lasers12 whose pump energy is coupled into the Er-doped optical fibre 11 viafirst and second wavelength division multiplexers 14 and 15. An inputsignal 16 is coupled into the Er-doped optical fibre 11 via the firstwavelength division multiplexer 14, is amplified by the Er-doped opticalfibre 11, and is coupled to an output port 17 via the second wavelengthdivision multiplexer 15. The Er-doped optical fibre 11 is a single modeoptical fibre containing the erbium doping within its core. Thus thesingle mode core guides both the signal 16 and the pump energy from thepump lasers 12.

As can be seen from FIG. 1 a conventional amplifier does not offer anypump redundancy: if one of the pump lasers 12 fails, the gain of theamplifier 10 drops significantly. Moreover today's semiconductor-diodepump lasers can deliver no more than about 200 mW of power into asingle-mode fibre. This limited pump power imposes limits on achievablesignal power that can be output from the amplifier 10.

Recently Spectra Diode Labs Inc of San Jose, Calif., USA has developed awavelength-multiplexed pump module. This source combines the output ofup to four pump modules to obtain up to 500 mW of pump power [see forexample Spectra Diode Labs product catalogue, part # SDLO WM4]. In thismodule four individual pumps are spectrally separated by 5 nm so thatall pumps are within the erbium absorption band. This method offers someprotection against failure of pump diodes, but the module itself isquite expensive and cannot be easily upgraded to a greater number ofpumps due to the relatively narrow absorption peak of erbium ions insilica glass in the wavelength region around 980 nm [see for example E.Snitzer, H. Po, R. Tumminelli, P. Hakimi, U.S. Pat. No. 4,815,079].

An approach to increase the signal power available from an opticalamplifier is suggested in V. P. Gapontsev and I. Samartsev, WO95/10868:the use of a so-called double-clad fibre for cladding-pumping. Theprior-art principle is shown in FIG. 2. A double-clad fibre 21 comprisesa core 23, a primary (inner) cladding 24, and a secondary (outer)cladding 25. Pump light 22 is launched directly into the primarycladding 24, which is capable of guiding light due to the presence ofthe secondary cladding 25 which has a lower refractive index than therefractive index of the primary cladding 24. The secondary cladding 25is typically a polymer coating that is applied during the manufacture ofthe double-clad fibre 21.

In this type of optical amplifier, the core 23 is usually doped withrare-earth ions, while the pump light 22 is launched into the primarycladding 24. Typically, the diameter of the core 23 is in the region of5-25 μm and the refractive index difference between the core 23 and theprimary cladding 24 is between 3×10⁻³ and 1×10⁻². A relatively largediameter of the primary cladding 23 allows the use of broad stripe,semiconductor-laser pump diodes with 1 to 5 W of pump power obtainedfrom a 100 μm×1 μm stripe. The result is that cladding pumped fibreamplifiers based on the double-clad fibre 21 can deliver much higheroutput power in comparison with the conventional, core-pumped amplifier10.

FIG. 3 shows the prior art double-clad fibre 21 configured as a fibrelaser 31. In this configuration, a pump beam 32 is launched through adichroic mirror 33 into the double-clad fibre 21. A high-reflectivitymirror 34 is used to reflect back both pump and signal. The resultingsignal 35 from the fibre laser 31 is separated from the pump beam 32 bythe dichroic mirror 33.

Cladding-pumped optical amplifiers can be constructed using dichroicmirrors in similar configurations to the fibre laser 31 shown in FIG. 3.However, a problem associated with this and many other experimentaldevices based on double-clad fibres 21 is that it is difficult to accessthe signal of the fibre laser or amplifier because it is necessary touse one or both ends of the double-clad fibre 21 for launching pumppower. A further limitation is that at most two pump diodes can belaunched into the double-clad fibre 21 unless complex polarization orwavelength division multiplexed schemes are used. Accordingly, simpleend-pump prior art configurations of fibre lasers and amplifiers such asshown in FIG. 3 offer limited pump redundancy.

FIG. 4 shows a prior art multimode fibre coupler 40. An auxiliary pumpoptical fibre 41 is used to launch pump light 42 into a double-cladfibre 21 that is doped with rare-earth ions in its core 23. Uponpumping, the rare earth ions become excited and the core becomesamplifying. The resulting amplified spontaneous emission provides anoutput signal 43 from both ends of the double-clad fibre 21. A mainadvantage of this scheme is that both ends of the double-clad fibre 21are now accessible for launching and out-coupling signal power forsignal manipulation [see for example D. J. DiGiovanni, R. S. Windeler,A. M. Vengsarkar, J. L. Wagener, U.S. Pat. No. 5,802,236]. Furthermore,this scheme can offer protection against pump diode failure usingmultiple auxiliary pump optical fibres 41 as shown in the prior artdiagram of FIG. 5. The solution shown in FIGS. 4 and 5 is thereforehighly flexible allowing many configurations of fibre lasers andamplifiers to be constructed.

However, a major problem associated with the use of the multimode fibrecoupler 40 is it is necessary to remove the secondary cladding 25 inorder to attach the auxiliary pump optical fibre 41 to the double-cladfibre 21. This is a difficult process, resulting in a numerical aperture(NA) mismatch and potential reliability issues.

An important parameter for double-clad fibres is the numerical aperturefor the inner cladding. The numerical aperture depends on the refractiveindices of the inner and outer cladding. The refractive index of theinner cladding is determined by choice of material, a choice thatdepends on several other parameters besides the refractive index. Fusedsilica is one preferred material for the inner cladding, with arefractive index of 1.45. This is relatively low refractive index, whichmakes it difficult to find a material for the outer cladding with adesired, much lower, refractive index. A polymer cladding is onepossibility. For instance, silicone rubber would lead to an NA for theinner cladding of 0.4.

While 0.4 is a relatively large numerical aperture, polymer coatings cansuffer from rather low power handling capability and a relatively highloss. Reliability is also a concern. These issues become more criticalthe higher output power is required. An all-glass structure with a glassouter cladding is preferred from these points of view. However, even alow-index glass like fluorosilicate leads to an NA of only 0.25 with apure fused silica inner cladding. This low NA imposes certainlimitations on the performance of cladding pumped fibre lasers andamplifiers. The main constraint arises from the brightness theorem. Thisis a fundamental governing law that dictates system design. It statesthat the brightness J of an optical system can not be increased bypassive means, and this can be written as

$J_{pump} = {{\frac{P_{pump}}{{A_{eff}^{p} \cdot \Omega_{x}}\Omega_{y}} \geq J_{fibre}} = \frac{P_{fibre}}{A_{eff}^{f} \cdot {NA}^{2}}}$where A_(eff) ^(j) is an effective cross section of the pump diode orfibre, Ω_(i) is the pump diode divergence in fast and slow directionsand NA is the numerical aperture of the fibre for the pump beam.

Currently, laser diodes offer brightness in the region of 0.3 W/μm²(assuming 2 W, 100 μm×1 μm stripe and 0.7×0.1 NA). For reliabilityreasons the pump diodes are often down-rated by a factor of 4, so thatthe real brightness is in the region of 0.1 W/μm². Thus for example, a10 W fibre laser system requires about 50 W of pump power delivered by25 to 50 pump diodes (assuming 20% overall optical efficiency). If thesystem is based on a fibre with inner and outer claddings of differentglass materials with an NA of 0.25 for typical choices of glasses, thefibre outer diameter (OD) should be greater than 100 μm. Thus even amodest 10 W of output power would require a fibre OD greater than 100μm. For 200 W lasers the fibre OD should be in the region of 1 mm. Thiscreates problems in that the large inner cladding reduces theinteraction between the pump beam and the core. Therefore, even longerfibres are required.

A typical double-clad fibre with a silica inner cladding according tothe prior art can either have a low-index polymer coating with a high NAand low power handling, or a relatively higher index glass outercladding with a low NA but that can handle high powers well. Both ofthese options impose limits on the design and performance of high-powerfibre lasers.

Recently, a new approach to an all-glass structure with a high NA hasbeen demonstrated [D. J. DiGiovanni, R. S. Windeler, A. M. Vengsarkar,J. L. Wagener, U.S. Pat. No. 5,802,236]. The basic idea is to let thepump waveguide be formed by glass surrounded by air, with NA>1. Thestructure is then supported by a thin outer glass shell that surroundsthe fibre and runs along its length. However, this type of fibre is notreadily used together with the pump-couplers of FIGS. 4 and 5. Moreover,because of the large difference in refractive indices between air andglass, it is quite difficult to make a pump waveguide with sufficientlylow loss.

It is also difficult to use the pump couplers in FIGS. 4 and 5 withall-glass double-clad fibres: it is difficult to remove the glass outercladding, as is required for efficient coupling. Even the polymer-cladfibre presents problems for the pump coupler: while it is easy to removethe outer cladding to expose the inner cladding for pump coupling, thecoating at the point on the fibre where the coating reappears willabsorb a large fraction of the power lost in the device, since at thispoint, any mode that is guided by the glass-air interface, but not bythe glass-polymer interface, will be absorbed. This can cause thecoating to burn, and therefore limits the power that the coupler canhandle.

With reference to FIG. 7, there is provided an optical fibre arrangement70, which optical fibre arrangement 70 comprises at least two opticalfibre sections 71, 72, the optical fibre sections 71, 72 each having anoutside longitudinally extending surface 73, and the outsidelongitudinally extending surfaces 73 being in optical contact with eachother.

By “optical contact” we mean that light propagating in the near surfaceregion in one of the adjacent optical fibre sections 71 can penetrateinto the near-surface region of the other adjacent optical fibre section72. This will clearly not be the case if one of the optical fibresections is coated with a typical coating such as an acrylate orsilicone compound. The optical fibre sections 71, 72 can be of constantcross-section along their length.

The optical fibre sections 71, 72 can comprise a core and at least onecladding. The core can be circular or non-circular. The core can be inthe located in the center of the cladding or offset from the center. Thecladding can be circular or non-circular. One or more of the opticalfibre sections 71, 72 can comprise a glass rod that can be silica orsoft glass.

The optical fibre sections 71, 72 can be constructed from the sameoptical fibre 81 as shown in FIG. 8, or from different optical fibres91, 92 as shown in FIG. 9.

FIG. 10 depicts a cross-section through an optical fibre arrangement inwhich a first optical fibre 101 having a core 103 and a cladding 104 isin optical contact with a second optical fibre 102 having only acladding 105. The first optical fibre can be a single-mode or multi-modeoptical fibre and the second optical fibre 102 can be silica rod. Theoptical fibre arrangement is preferably constructed from freshly drawnoptical fibre. FIG. 11 depicts a similar optical fibre arrangement inwhich a first optical fibre 111 is fused to a second optical fibre 112.The optical fibres are preferably fused during the fibre drawing process(in which the optical fibre is manufactured) or subsequently. The firstand second topical fibres 111, 112 can be single-mode or multi-modeoptical fibres.

FIGS. 12 to 20 show alternative optical fibre arrangements in which atleast one first optical fibre 120 is optically connected to at least onesecond optical fibre 121. Each of the first and second optical fibres120, 121 can have a circular cross section or a non circular crosssection, and either or both the first and second optical fibres 120, 121can each contain waveguiding cores that can be situated in the center ofthe optical fibre 120, 121 or offset from the center.

The first and second optical fibres 120, 121 can be formed from a glassselected from the group consisting of silica, doped silica, silicate,phosphate, and soft glass. The first optical fibre 120 can also be anamplifying optical fibre doped with rare-earth ions.

The amplifying optical fibre preferably has a single multimode claddingand a waveguiding core. The core and/or cladding can comprise at leastone rare earth dopant comprising Ytterbium, Erbium, Neodymium,Praseodymium, Thulium, Samarium, Holmium, Dysprosium or it can be dopedwith a transition metal or a semiconductor. The core and/or cladding canbe co-doped with Erbium/Ytterbium. The core and/or cladding can be dopedwith germanium, phosphorous, boron, aluminium and/or fluoride. The corediameter can be substantially in the range of 2 μm to 100 μm. Thecladding area can be at least 10 to 1000 times larger then the crosssectional area of the core.

More than one amplifying optical fibre can be included in the opticalfibre arrangement 70, each one of the amplifying optical fibrescontaining the same dopants or different dopants. The second opticalfibre 121 can be a pump optical fibre, the pump optical fibre being inoptical contact with the amplifying optical fibre along at least aportion of its length.

FIG. 20 depicts a cross-section through an amplifying opticalarrangement 70 in which the first optical fibre 120 is a so-called“holey fibre” 201 (or “photonic bandgap fibre”) that comprises awaveguide constructed from a lattice of azimuthal holes 202 extendingalong the axis of the fibre 201. The holey fibre 201 can be doped withone or more of the rare earth dopants comprising Ytterbium, Erbium,Neodymium, Praseodymium, Thulium, Samarium, Holmium, Dysprosium, or itcan be doped with a transition metal or a semiconductor. The core and/orcladding can be co-doped with Erbium/Ytterbium. The lattice can be aregular lattice or an irregular lattice.

The second optical fibre 121 can be a pump optical fibre, the pumpoptical fibre being in optical contact with the holey fibre 201 along atleast a portion of its length. The amplifying optical arrangement cancontain a single second optical fibre 121 or a plurality of secondoptical fibres 121. The second optical fibre 121 can also be a pumpoptical fibre, the pump optical fibre being in optical contact with theamplifying optical fibre along at least a portion of its length.

FIG. 21 depicts an optical fibre arrangement in which the optical fibresections 71, 72 are twisted about each other. The term “twisted” isbeing used here in a very general sense as is illustrated in FIGS. 22and 23 where a pump optical fibre 221 is shown twisted around at leastone amplifying optical fibre 222. The pump optical fibre 221 is shown ashaving a diameter very much less than that of the amplifying opticalfibre 222. The amplifying optical fibres 222 in FIG. 23 are shown inoptical contact with each other and with the pump optical fibre 221.

The amplifying optical fibre 222 preferably has a single multimodecladding and a waveguiding core. The core and/or cladding can compriseat least one rare earth dopant comprising Ytterbium, Erbium, Neodymium,Praseodymium, Thulium, Samarium, Holmium, or Dysprosium, or it can bedoped with a transition metal or a semiconductor. The core and/orcladding can be co-doped with Erbium/Ytterbium. The core and/or claddingcan be doped with germanium, phosphorous, boron, aluminium and/orfluoride. The core diameter can be substantially in the range of 2 μm to100 μm. The cladding area can be at least 10 to 1000 times larger thenthe cross sectional area of the core. The rare earth dopant can bedisposed in the core, in the cladding, in regions in the core and thecladding, or in a ring around the core.

More than one amplifying optical fibre 222 can be included in theoptical fibre arrangement 70, each one of the amplifying optical fibres222 containing the same dopants or different dopants.

The amplifying optical fibre 222 can comprise a waveguide constructedfrom so-called “holey fibre” or “photonic bandgap fibre” and can bedoped with one or more of the rare earth dopants listed above.

The pump optical fibre 221 can have a substantially uniform refractiveindex across its cross-section and can be drawn from a silica rod.

FIG. 24 depicts a first optical fibre 241 and six second optical fibres242 twisted together in such a way that the outside surface of at leasttwo adjacent fibres are in optical contact along at least a respectiveportion of the length of the optical fibre arrangement. The firstoptical fibre 241 can be the amplifying optical fibre 222, and thesecond optical fibre 242 can be the pump optical fibre 221.Alternatively the first optical fibre 241 can be the pump optical fibre221 and the second optical fibre 242 can be the amplifying optical fibre222.

FIG. 25 depicts yet another arrangement in which four first opticalfibres 241 and three second optical fibres 242 are straight, and twothird optical fibres 243 are twisted around the first and second opticalfibres 241 and 242.

FIG. 26 depicts an optical fibre arrangement in which the optical fibresections 71, 72 are surrounded by a coating material 262 along a lengthof the optical fibre arrangement. For clarity, the optical fibresections 71, 72 are shown extending and separating on either side of thecoating material 262. The coating material can be a polymer with arefractive index less than the refractive index of a cladding materialof at least one of the optical fibre sections 71, 72. The coatingmaterial can be silicone rubber. The optical fibre sections 71, 72 canbe a section of the amplifying optical fibre 222 and/or a section of thepump optical fibre 221.

Advantageously, one of the optical fibre sections 71, 72 can beindividually separated by pulling it from the remaining optical fibresection or sections 71, 72. This is a feature which can be particularlyuseful in the design and manufacture of a range of optical fibreamplifiers and lasers, since it simplifies the problem of coupling ofmultiple pump sources to an optical fibre amplifier or fibre laser. Italso enables parallel (i.e., multi-channel) optical amplifiers to beconstructed, can have major cost and reliability advantages over theprior art.

FIG. 27 depicts an apparatus for manufacturing long lengths of anoptical fibre arrangement in the form of an optical fibre cable 277. Afirst and second optical fibre preform 271, 272 is placed in a chuck 273on a fibre drawing tower 270 and lowered into a furnace 274. A first andsecond optical fibre 275, 276 is drawn from the first and second opticalfibre preforms 271, 272, twisted together, and wrapped around a drawingdrum 278, which rotates at a given speed. The first and second opticalfibres 275, 276 are drawn by rotating the drawing drum 278, and rotatingthe first and second optical fibre preforms 271, 272 while lowering thefirst and second optical fibre preforms 271, 272 into the furnace. Thefirst and second optical fibres 275, 276 can be twisted together byrotating the chuck 273.

FIG. 28 depicts a similar apparatus, which includes a coating cup 281containing a coating material 282 that is cured in a curing apparatus283 during the fibre drawing process. The curing apparatus can be afurnace or a UV curing chamber depending on the type of coating materialbeing applied.

The invention therefore provides the following method for manufacturingan optical fibre arrangement comprising: providing a first and secondoptical fibre preform 271, 272 having optical fibres 275, 276; mountingthe first and second optical fibre preforms 271, 272 in a chuck 273 on afibre drawing tower 270; drawing a first and second optical fibre 275,276 from the first and second optical fibre preforms 271, 272 under adrawing tension and at a drawing speed; and twisting the first andsecond optical fibre 275, 276 during the drawing process; the drawingtension and the drawing speed being selected such that the outsidesurface of the first and second optical fibres 275, 276 are in opticalcontact along at least a respective portion of their lengths. The firstand second optical fibres 275, 276 can be passed through a coating cup281 during the manufacturing process.

It is to be appreciated that it will not always be convenient orpracticable to manufacture an optical fibre arrangement directly duringthe fibre drawing process. The above method can be modified by usinguncoated optical fibre, which can be unwound from drums. Such a methodcomprises the following steps: providing a first and second opticalfibre 275, 276; pulling the first and second optical fibres 275, 276under a drawing tension and at a drawing speed; and twisting the firstand second optical fibres 275, 276 during the drawing process; thedrawing tension and the drawing speed being selected such that theoutside surface of the first and second optical fibres 275, 276 are inoptical contact along at least a respective portion of its length. Thefirst and second optical fibres 275, 276 can be passed through a coatingcup 281 during the manufacturing process.

It can be convenient to apply a coating to the first and second opticalfibres 275, 276 when they are first manufactured that can be removedimmediately prior to manufacturing the optical fibre arrangement. Careneeds to be taken to ensure that the surface of the optical fibre is notdamaged during such removal of the coating.

Referring again to FIG. 26, the coating material 262 can also be a glasshaving a refractive index less than the refractive index of the pumpoptical fibre 221. The glass can be applied using a sol-gel process. Theglass may be silica glass, a doped silica glass, or a soft glass.Advantageously, the glass can be leached away for example by acidetching to expose the pump optical fibre 221 and the amplifying opticalfibre 222 for subsequent connection to optical devices.

FIG. 29 depicts a cross-section of an optical fibre arrangement in whichthe first and second optical fibre sections 71, 72 are joined togetherby an optical glue 291. An optical glue in this context means that in anarrangement where two adjacent fibres are in close proximity butseparated by a thin layer made of optical glue, then light can propagatefrom one of the fibres to the other through the optical glue layer.

FIG. 30 depicts an amplifying optical device 300 comprising the opticalfibre arrangement 70 and a pump source 302. The optical fibrearrangement 70 can be an amplifying optical fibre 222, and a pumpoptical fibre 221. The pump optical fibre 221 and the amplifying opticalfibre 222 are shown in optical contact with each other and thus pumpenergy propagating along the pump optical fibre 221 couples through tothe amplifying optical fibre 222. The pump optical fibre 221 can have asmall diameter than the diameter of the amplifying optical fibre 222.The pump optical fibre 221 can have a reflecting device 225 deposited orpositioned at or near its end face. The reflecting device 225 can be anoptical grating, a mirror, or a loop of optical fibre connected to afibre coupler. The amplifying optical fibre 222 can be a single-claduncoated optical fibre. The pump optical fibre 221 can also be twistedaround the amplifying optical fibre 222 as shown in FIG. 31.

An optical amplifier based upon the amplifying optical device 300preferably includes at least one optical isolator orientated to amplifyan optical signal at the input of the amplifier and a filter to filterout amplified spontaneous emission at the output of the amplifier. Byamplifying optical device we mean an optical amplifier, a poweramplifier, a laser, a broadband source of amplified spontaneousemission.

FIG. 32 depicts an amplifying optical device comprising an optical fibrearrangement 70 in which the optical fibre sections 71, 72 comprise oneamplifying optical fibre 222 and two pump optical fibres 221, theamplifying optical fibre 222 and the pump optical fibre 221 beingsurrounded by the coating material 262. Each end of the pump opticalfibres 221 is shown connected to a separate one of the pump sources 302.The figure illustrates the advantage of being able to individuallyseparate the pump optical fibres 221 from the amplifying optical fibre222. This configuration is especially useful for designing a high-poweroptical amplifier and moreover offers pump redundancy.

FIG. 33 depicts an amplifying optical device comprising two amplifyingoptical fibres 222. One of the notable aspects of this embodiment isthat the pump energy being supplied by the pump optical sources 302 isshared by more than one of the amplifying optical fibres 222 by virtueof the optical contact between the pump optical fibres 221 and theamplifying optical fibres 222. Further amplifying optical fibres 222 canbe added and the amplifying optical device used as a parallel (ormulti-channel) optical amplifier. Such an amplifier tends to equalizethe output power provided by each of the amplifying optical fibres 222by virtue of the shared pump energy between the amplifying opticalfibres 222. Advantageously, each amplifying optical fibre 222 is capableof amplifying individual signals having the same or differentwavelengths with low cross-talk and low interference between signalsbeing amplified by different ones of the amplifying optical fibres 222.

FIGS. 34 to 36 depict an amplifying optical device comprising aplurality of pump optical fibres 221 and a plurality of amplifyingoptical fibres 222, in which at least one end of the pump optical fibres221 are connected to a pump source 302 supplying pump energy, and inwhich the optical fibre arrangement 70 is configured such that a portionof the optical energy guided by each of the pump optical fibres 221 iscoupled into at least one of the amplifying optical fibres 222, and inwhich at least two amplifying optical fibres 222 are connected together.The amplifying optical devices shown in FIGS. 34 to 36 can comprise thecoating 262.

The differences between the embodiments shown in FIGS. 34 to 36 is inthe number of connections between the amplifying optical fibres 221 andin the connection of the pump optical fibres 221 to the pump sources302. In FIG. 34, the pump optical fibres 221 are joined together at oneend of the amplifying optical device, whereas in FIGS. 34 and 36, eachof the ends of the pump optical fibres 221 are connected to differentpump sources 302. The advantage of joining the pump optical fibres 221together is to achieve greater absorption of pump power. This should becompared with the advantages of pumping each of the optical fibres 221from both ends, which are greater pump redundancy, increased saturationpower and increased optical gain. The flexibility in the options forpumping the optical fibre arrangement 70 is an advantage that isachieved with the present invention. The amplifying optical device shownin FIG. 35 can be configured with a single pump source 302, which willreduce the pump redundancy and the saturated power.

The amplifying optical device shown in FIG. 34 is a parallel (ormulti-channel) optical amplifier that in effect comprises severalamplifiers that share pump energy derived from common pump sources 302,wherein amplification in each amplifier is achieved in more than onepass through the optical fibre arrangement 70.

FIGS. 35 and 36 depict amplifying optical devices wherein all theamplifying optical fibres 222 are joined together in series, theconfigurations being high power optical amplifiers. The amplifyingoptical fibres 222 in FIG. 35 are connected such that an optical signalpasses in both directions through the optical fibre arrangement 70 whilebeing amplified, whereas the amplifying optical fibres 222 in FIG. 36are connected such that an optical signal passes in the same directionthrough the optical fibre arrangement 70 while being amplified. Incertain instances it can be preferable to configure the amplifyingoptical device as shown in FIG. 36—for example it can provide a lowernoise figure. In other cases it is preferable to configure theamplifying optical device as shown in FIG. 35—for example it can providehigher gain.

The embodiments are similar to the parallel optical amplifier shown inFIG. 34. The parallel optical amplifier of FIG. 34 can be configured asthe high-power optical amplifier of FIG. 36 simply by connecting theamplifying optical fibres 222 together. It will therefore be appreciatedthat the parallel optical amplifier of FIG. 33 provides significantflexibility in its use. These configurations also further illustratethat the advantage of being able to individually separate the pumpoptical fibres 221 and the amplifying optical fibres 222 clearlyincreases as the numbers of pump optical fibres 221 and amplifyingoptical fibres 222 increases.

FIGS. 33 to 36 indicate the use of multiple pump optical sources 302.These embodiments can operate with a single pump optical source 302.However, the use of multiple pump optical sources 302 provides pumpredundancy and is therefore preferable in certain applications whereadditional reliability is desired.

FIG. 37 depicts an amplifying optical device 370 which includes anoptical element 371 inserted along the length of the amplifying opticalfibre 222, the optical element 371 comprising at least one of apolarizer, an isolator, a circulator, a grating, an optical fibre Bragggrating, a long-period grating, an acousto-optic modulator, anacousto-optic tunable filter, an optical filter, a Kerr cell, a Pockelscell, a dispersive element, a non-linear dispersive element, an opticalswitch, a phase modulator, a Lithium Niobate modulator, or an opticalcrystal.

The amplifying optical device 370 can be considered to be either asingle amplifying optical device containing the optical element 371, ortwo amplifying optical devices connected together via the opticalelement 371. The amplifying optical device 370 can also comprise thecoating 262.

The amplifying optical device 370 can be constructed from an opticalfibre arrangement 70 in which the amplifying optical fibre 222 can beindividually separated by pulling from the remaining pump optical fibres221, thus facilitating the insertion of the optical device 370. This isan advantageous feature that has particular benefit in the design andmanufacture of a range of optical fibre amplifiers and lasers, since itsimplifies the problem of adding more flexibility into the design ofoptical amplifiers and lasers.

FIG. 38 depicts an embodiment of the amplifying optical device of FIG.37 in which the optical element 371 is an optical fibre Bragg grating382, and optical energy propagating in the amplifying optical fibre 222is coupled into the optical fibre Bragg grating 382 via an opticalcirculator 381. The optical fibre Bragg grating 382 can be one or bothof a gain-flattened grating and a dispersion compensating grating. Theseembodiments of the present invention have particular application intelecommunication systems. By gain flattening, we mean that the fibregrating compensates for the spectral variation in the optical gainprovided by the amplifying optical fibre 222. FIG. 39 depicts anamplifying optical device in which the optical element 371 connects twoamplifying optical fibres 282.

FIG. 40 depicts an amplifying optical device in which the opticalelement 371 and a reflecting device 401 is configured to reflect opticalenergy being emitted from the amplifying optical fibre 222 back into thesame amplifying optical fibre 222. The reflecting device 401 can be amirror or an optical fibre Bragg grating. It is to be appreciated thatthe amplifying optical device shown in FIG. 40 can be configured as alaser by adding the second reflecting device 401 as shown. The laser canbe configured as a Q-switched or a mode-locked laser.

FIG. 41 depicts an amplifying optical device comprising a singleamplifying optical fibre 222 configured as a coil 411 such that at leasttwo adjacent turns of the single amplifying optical fibre 222 are inoptical contact with each other. Optical pump power 412 can be coupledinto the amplifying optical device by side illumination from at leastone optical pump source 302 as shown in FIG. 41, or by utilizing one ofthe prior art methods described in prior art FIGS. 1 to 5.

The amplifying optical fibre 222 is preferably an unclad optical fibrethat can be either single-mode or multimode, and have a circular ornon-circular cross-section.

The coil 411 can be supported by at least one support 415. The support415 can be a ceramic, glass or silica rod, tube, cylinder, or bead,epoxied or otherwise bonded to the coil 411. The support 415 can be asupport means. The coil 411 can be enclosed within an enclosure, whichcan be sealed and evacuated, or filled with inert gas such as nitrogenor argon.

FIG. 42 depicts an amplifying optical device comprising a singleamplifying optical fibre 222 configured as a coil 421 such that at leasttwo adjacent turns of the single amplifying optical fibre 222 are inoptical contact with each other, and including at least one pump opticalfibre 221 disposed with respect to the coil 421 of amplifying opticalfibre 222 so that the pump optical fibre 221 touches the amplifyingoptical fibre 222 along at least a respective portion of its length. Asshown in FIG. 43, the amplifying optical device can comprise a pluralityof pump optical fibres 221 to form a coil 431. The amplifying opticalfibre 222 and the pump optical fibres 221 are shown laying in aclockwise direction.

The amplifying optical device can comprise a plurality of amplifyingoptical fibres 222 as shown in FIG. 44. This is conveniently constructedby twisting the amplifying optical fibres 222 and at least one pumpoptical fibre 221 together to form an interim cable 442, and coiling theinterim cable 442 to form a coil 441. The amplifying optical device ofFIG. 44 is a parallel optical amplifier with the performance advantagesof the amplifying optical device described with reference to FIG. 33.

The coils 411, 421, 431 and 441 can be potted in a polymer 443 as shownin FIG. 44. The polymer 433 preferably has a refractive index lower thanthe refractive indices of the claddings of the amplifying optical fibres222 and the pump optical fibre 221. The polymer 443 can be a siliconerubber.

The pump optical fibre 221 depicted in FIGS. 42 to 44 can have adiameter much less than the diameter of the amplifying optical fibre222. Advantageously, the pump optical fibre 221 can be disposed ininterstitial gaps between turns of the amplifying optical fibre 222 asillustrated in FIG. 45. The pump optical fibre 221 shown in FIG. 45 caneither be a single pump optical fibre or be many pump optical fibres—aconfiguration that has particular applicability to high-power amplifiersand lasers as well as providing a means to achieve pump redundancy.

The pump optical fibre 221 can be formed from a material having a lowermelting point than the material of the amplifying optical fibre 222.

The coils 421, 431 and 441 in which one or more pump optical fibres 222are attached can be considered to be a pumped coil. The number of pumpoptical fibres 221 can be between 1 and 100, or even higher forapplications involving amplifiers and lasers requiring high poweroutputs (>1 W to 5 W). The pump optical fibre 221 is preferably amultimode fibre fabricated either from silica or soft glass. For certainapplications, it is convenient to have the pump optical fibre 221smaller than the amplifying optical fibre 222, i.e., in the range 5 μmto 100 μm. For other applications, the pump optical fibre 221 should beof a comparable size or even much larger than the amplifying opticalfibre 222. For example, when coupling to a diode bar, the pump opticalfibre 221 can conveniently be in the region 100 μm to 1000 μm—the largerdimension representing a glass rod which can be moulded into the pumpedcoil. The coil turns in the pumped coil can be melted to each other. Thediameter of the pumped coil can be in the range 10-1000 times greaterthan the diameter of the amplifying optical fibre 222.

FIG. 46 depicts a coil 461 comprising amplifying optical fibre 222 woundon a light transmitting former 462. Also shown is a pump arrangement 463for launching pump light 464 into the former 462. The pump arrangement463 can be the pump source 302. The amplifying optical fibre 222 has alongitudinally extending outside surface that is in optical contact withthe former 462 along at least a portion of the longitudinally extendingoutside surface. The former 462 can be a glass tube, a glass rod, aglass cylinder, or a glass hoop.

FIG. 47 depicts a pump optical fibre 221 in optical contact with theformer 462, the pump optical fibre 221 being connected to a pump source302. In use, pump light will be coupled from the pump optical fibre 221into the amplifying optical fibre 222 via the former 472.

In configurations wherein the former 462 is a hoop, the coils can beconveniently wound around the hoop in a toroidal winding. The glass canbe a soft glass, or can be silica or doped silica glass. Preferably, therefractive index of the glass is substantially the same as therefractive index of the cladding of the amplifying optical fibre 222 andthe pump optical fibre 221.

FIG. 48 depicts a laser 480 constructed from an amplifying opticaldevice 481 by providing an optical feedback arrangement 482 forpromoting light generation within the laser. The amplifying opticaldevice 481 can be one of the amplifying optical devices described withreference to FIGS. 30 to 47. The optical feedback arrangement 482 cancomprise two reflecting devices comprising at least one of a mirror, adichroic mirror, a coupler, an optical fibre coupler, and/or an opticalfibre Bragg grating. The optical feedback arrangement 482 can beconfigured such that the laser 480 is a ring laser 490 as shown in FIG.49. Here, the optical feedback arrangement 482 is shown as a coupler 491to provide two output ports 492. If unidirectional operation isrequired, an optical isolator can be added into the ring according toprior art.

FIG. 50 depicts a preferred embodiment for an optical amplifier 500configured as a parallel optical amplifier, which will be referred to inthe following description to demonstrate the advantages of such aparallel amplifier in optical networks. The amplifier 500 isparticularly advantageous for amplifying optical devices such as shownin FIGS. 30 to 47. The amplifier 500 comprises at least one pump source302 for supplying pump energy, and a plurality of amplifying opticalfibres 222. The amplifier 500 preferably comprises a plurality of pumpoptical fibres 221—although as seen in FIG. 41, this feature is notstrictly necessary, and is not meant to limit either this embodiment orthe embodiments that will be described in the following figures wherethe amplifier 500 is referenced. The underlying feature is that the pumpenergy provided by the pump source 302 (or any other pump arrangement)is shared between the plurality of the amplifying optical fibres 222 byvirtue of the optical contact of the optical fibre sections 71,72 (notshown in FIG. 50). The amplifier 500 can either be used for single-passamplification, or for multi-pass amplification by connecting oneamplifying optical fibre 222 to another amplifying optical fibre 222 asdescribed in the description relating to FIGS. 34 to 36. The amplifier500 provides a number of substantially independent amplificationchannels that can simultaneously amplify signals at the same wavelengthor at different wavelengths.

The pump source 302 preferably contains at least one semiconductor laserdiode, and there is preferably more than one pump source 302 connectedto each of the ends of the pump optical fibre 221. The semiconductorlaser diode can be a broad stripe laser diode or a diode bar. There ispreferably more than one pump optical fibre 221 connected to additionalpump sources 302. These features are preferred to increase pumpredundancy, to increase the saturated power available from eachamplifying optical fibre 222, and to amortize the investment of therelatively expensive semiconductor laser diodes over several amplifyingoptical fibres 222. This latter feature is especially relevant inapplications requiring low-cost amplification, for example forapplication in metropolitan areas.

FIG. 6 schematically illustrates a prior art technique for launchingpump light from a laser diode 61 into the pump optical fibre 221. Thepump light is emitted from an emission stripe 62 of the laser diode 61.As it has a much larger divergence in one dimension (vertically alongthe page as drawn), a cylindrical lens 63 formed as a piece of opticalfibre is used to converge the pump light in this direction. The pumplight is then launched into a fibre 64 in which at its end has adiameter of about 140-300 μm, but which is then tapered down in a taper65 to about 80 μm for use in coupling to the pump optical fibre 221. Thefibre 64 and the taper 65 can be constructed from the same opticalfibre, or different optical fibres, and can be a part of the pumpoptical fibre 221. This technique can also be used to launch light intoother types of optical fibre.

FIG. 51 depicts an amplifier 510 comprising the amplifier 500, at leastone input fibre 511, and a first multiplexer 512 connected to the inputfibre 511. The amplifying optical fibres 222 are connected to the firstmultiplexer 512. The first multiplexer 512 can be a coupler dividing thepower essentially equally between its outputs. The coupler can beconstructed from optical fibre couplers or can be a planar-opticaldevice having a single input and multiple outputs.

The first multiplexer 512 can be a wavelength division multiplexer as anarrayed waveguide grating AWG. The first multiplexer 512 can also be anadd mulitplexer, a drop multipexer, or an add-drop multiplexerconstructed from thin-film filters and/or optical fibre gratings. Thefirst multiplexer 512 can be used to separate out wavelength channelsinput by the input fibre 511 such that each amplifying optical fibre 222amplifies either different wavelength channels or groups of differentwavelength channels. The separate wavelength channels can be combinedinto a single output port 513 using a coupler 514 as shown in FIG. 52.The coupler 514 can be a planar-optics coupler, one or more opticalfibre couplers, a wavelength division multiplexer, an add mulitplexer, adrop multipexer, or an add-drop multiplexer constructed from thin-filmfilters and/or optical fibre gratings. FIG. 61 depicts eight wavelengthchannels 611 to 618 output by an amplifier. Wavelength channels 611 and612 are adjacent and so are wavelength channels 613 and 614. It ispreferable that each amplifying optical fibre 222 amplifies only asingle one of the wavelength channels.

FIG. 52 depicts an amplifying optical arrangement comprising a pluralityof amplifiers 500 and a plurality of first multiplexers 512, andincluding a second multiplexer 521 connected to each first multiplexer312 and to an input port 522. The second multiplexer 521 can be aninterleaver that directs adjacent wavelength channels to different onesof the first multiplexers 512 and hence to different ones of theamplifiers 500 or to a coupler that divides the input power between thetwo input fibres 511. The configuration with the interleaver ispreferred. Thus referring to FIG. 61, it is preferred that one of theamplifiers amplifies channels 611, 613, 615 and 617 whilst the otheramplifier amplifies channels 612, 614, 616 and 618.

FIG. 53 depicts an amplifying arrangement comprising a plurality ofamplifiers 500 and an optical device 531, the amplifying arrangementbeing configured such that the optical device 531 is connected to theamplifiers 500. The figure depicts one of the pump optical fibres 221being shared by two of the amplifiers 500—thus saving on pump sources302.

The optical device 531 can be an optical router, an add-dropmultiplexer, an add multiplexer, a drop multiplexer, an optical switch,a polarize, an isolator, a circulator, a grating, an optical fibre Bragggrating, a long-period grating, an acousto-optic modulator, anacousto-optic tuneable filter, an optical filter, a Kerr cell, a Pockelscell, a dispersive element, a non-linear dispersive element, an opticalswitch, a phase modulator, a Lithium Niobate modulator, or an opticalcrystal. The optical device 531 can also be more than one of the abovedevices, either singly or in combination.

A preferred embodiment is where the optical device 531 is an opticalrouter which comprises an optical switch configured such that opticalsignals output from one of the amplifiers 500 are routed to at least twomore of the amplifiers 500.

The amplifying arrangements depicted in FIGS. 51 to 53 are particularlyuseful for reducing the granularity from an optical telecommunicationsnetwork. This granularity occurs when a signal at a remote location issuddenly turned on. This signal propagates through the optical networkand can induce instabilities in an amplifier. The ability to separateout the individual wavelength channels into individual wavelengthchannels or groups of wavelength channels, each being amplifiedseparately in the amplifier 500 reduces the cross-talk inherent andinstabilities which occur with prior art amplifiers, and does so in acost effective manner.

FIG. 54 depicts an optical network 540 comprising at least one firstoptical fibre 541 that can be configured in at least one ring 545. Thenetwork includes at least one multi-wavelength transmitter 542comprising a plurality of signal sources (not shown) that can bedistributed feedback lasers, either directly modulated or with externalmodulation. The multi-wavelength transmitter 542 outputs a plurality oftelecommunication signals 5402 into the first optical fibre 541 via amultiplexer 543 and/or a first add/drop multiplexer 544, eachtelecommunication signal 5402 having a different wavelength. Anamplifier 5403 is shown in the ring 545.

A second add/drop multiplexer 546 can be included to remove at least onetelecommunication signal 5402 from the first optical fibre 541 andtransmit the telecommunication signal 5402 via a second optical fibre547 to at least one location 5400. An amplifier 548, a coupler 549 and athird optical fibre 5401 can ay be included.

The telecommunication network 540 is not intended to be limited to thearrangement shown in FIG. 54. There are many different topologies andarchitectures being explored today, including ring architectures andmesh architectures. However, the telecommunication network will, for theforeseeable future, always include the multi-wavelength transmitter 542and the first optical fibre 541.

The amplifier 5403 can be the amplifying optical device shown in FIG. 38that includes an optical fibre Bragg grating to condition at least onetelecommunication signal 5402. The amplifier 5403 be the amplifier 500.The first add/drop multiplexer 544 can include an amplifier according toFIG. 51 or an amplifying arrangement according to FIG. 52 or FIG. 53.The second add/drop multiplexer 546 can ay include an amplifieraccording to FIG. 51 or an amplifying arrangement according to FIG. 52or FIG. 53.

The amplifier 548 can ay be an amplifying optical device according toany one of FIGS. 30 to 39, 41 to 44, 46, 47, an amplifier according toFIG. 50 or FIG. 51, or can include an amplifying arrangement accordingto FIG. 52 or FIG. 53. It will be noted that the amplifier 548 can berequired to boost the telecommunication signal 5402 significantly inorder that a signal with adequate signal to noise ratio is received atthe location 5400.

The invention therefore provides a method to reduce the granularity inan optical telecommunications network comprising providing at least oneof the amplifiers shown in FIG. 50 and FIG. 51, and/or at least one ofthe amplifying optical arrangements shown in FIGS. 52 or FIG. 53 in atleast one location within the network.

FIG. 55 depicts a power splitter 550 comprising at least one pump source302 and an optical fibre arrangement 70 comprising a plurality of pumpoptical fibres 221 each having an input 555 and an output 556. Theplurality of pump optical fibres 221 is configured in a coil 551,wherein at least one of the pump optical fibres 221 is connected to thepump source 302.

Provided that each of the pump optical fibres 221 has the same diameter,the optical power provided by the pump optical source 302 is dividedapproximately equally between the pump optical fibres 221 within thecoil 551. The optical power provided by each output 556 of the pumpoptical fibres 221 can be predetermined by selecting the relativediameters of the pump optical fibres 221.

The pump optical fibres 221 can be twisted or can be left untwisted. Thecoil 551 can be constructed by forming an interim cable 552. The coil551 can be potted in a polymer 443. The polymer 433 preferably has arefractive index lower than the refractive indices of the claddings ofthe pump optical fibres 221. The polymer 443 can be a silicone rubber.

FIG. 56 depicts a power splitter 560 comprising at least one pump source302 and at least one optical fibre arrangement 70 comprising a pluralityof pump optical fibres 221 each having an input 565 and an output 566,wherein at least one of the pump optical fibres 221 is connected to thepump source 302. By varying the length of the optical fibre arrangement70, the power splitting ratio at the output of the optical powersplitter 550 can be set to a predetermined value. The pump opticalfibres 221 can be twisted or can be left untwisted.

Advantageously, the optical fibre arrangement 70 can be constructed froman optical fibre arrangement in which a first optical fibre isindividually separable from a second optical fibre. For example, opticalfibre arrangements based on the optical fibre 277 or the optical fibre284. Such an approach provides an advantageous solution for sharing pumpenergy from a single pump source amongst a plurality of amplifiers,particularly since many optical power splitters can be fabricated from atypical production quantity of the optical fibre 277 and the opticalfibre 284.

FIG. 57 depicts optical pump power from a pump source 302 being dividedby a power splitter 571 connected to a plurality of optical amplifiers573 by output fibres 572. The output fibres 572 can be pump opticalfibres 221. The power splitter 571 can be the power splitter 550 or thepower splitter 560. The arrangement depicted in FIG. 57 provides acost-effective and reliable way of sharing output from a single pumpsource amongst several optical amplifiers.

Selected detailed examples will now be presented of how pump light canbe injected into amplifying optical fibres and converted into signallight. The results will be compared to a prior-art, double clad fibrelaser.

EXAMPLE I

This Example is based on the configuration depicted in FIG. 32. Theamplifying optical fibre 222 has an outer diameter (OD) of 200 μm and acore diameter of 10 μm. The core is single-moded at the signalwavelength and made of Er3+/Yb3+-activated aluminosilicate glass. Thepump absorption cross section at 980 nm is 20·10-25 cm2. The Yb3+concentration is 9000 particles per million (ppm). A 10 m long fibreabsorbs ˜90% of launched pump power. The pump power is provided by 4laser diodes with rated output power of 2 W. Using the simple launchingscheme shown in FIG. 32 one can launch nearly 90% of pump power into thefibre amplifier

Assuming 35% efficiency the saturated output power of the example is inthe region of 2.5 W. In many applications however required output poweris 1 W which can be achieved with only 2.9 to 3 W of pump power, i.e. byusing only two pump diodes. Thus by down-rating all four pump diodes to800 mW one can achieve the required level of the output power. In thecase of failure of one of the pump diodes, pump power from the rest isincreased to the level required to obtain a pre-determined level of theoutput power. Thus this system has protection against pump diodefailure.

EXAMPLE II

This Example is a mode-locked cladding pumped fibre laser withrepetition rate frequency in the region of 50-200 MHz. The laser isbased on the two fibre arrangement shown in FIG. 40. The amplifyingoptical fibre has an OD of 80 μm, core diameter of 15 μm and signal NAof 0.07. The core is single-moded at the signal wavelength and made ofYb3+-activated aluminosilicate glass. The pump absorption cross sectionat 980 nm is 20X·10⁻²⁵ cm2, which implies an Yb3+ concentration of 1000particles per million (ppm). A 1 m long fibre absorbs approximately 90%of launched pump power. The pump power is provided by 2 laser diodeswith rated output power of 2 W.

With an appropriate mode-locking technique (either passive or active)the laser is capable of generating 1 ps pulses at repetition rate of 100MHz and average power of 1 W and peak power in excess of 10 kW. Mirrors401 form an optical resonator for the signal.

An advantage of using this configuration is that the signal and pump arespatially separated and thus high pulse peak power will not result indamage of pump diodes.

EXAMPLE III

This Example is a multi-fibre arrangement including two or more pumpdiodes pumping simultaneously several amplifying optical fibres as shownin FIGS. 33 to 36. The amplifying optical fibres have an outer diameter(OD) of 100 μm and a core diameter of 10 μm. The core is single-moded atthe signal wavelength and made of Er3+/Yb3+-activated aluminosilicateglass. The pump absorption cross section at 980 nm is 20.10⁻²⁵ cm²,which implies an Yb3+ concentration of 9000 particles per million (ppm).A 5 m long fibre absorbs approximately 90% of launched pump power. Thepump power is provided by 4 laser diodes with rated output power of 2 W.Using the simple launching scheme shown in FIG. 6 one can launch nearly90% of pump power into the fibre amplifier. Assuming 35% efficiency thesaturated output power of the Example is in the region of 1 W from eachchannel. It should be understood that total amount of output poweravailable from all channels remains approximately the same so increasingthe number of channels will results in decreasing of the output powerfrom an individual channel. It should be also understood that it ispreferred in a transmission system that optical power in any one channelshould not exceed 10 to 15 mW to avoid non-linear effects. Thus if thenumber of doped fibres is equal to the number of channels, then theoutput power from the individual channels will be below 20 mW. Thepresent invention also makes it possible to increase the number ofamplifying optical fibres to 16 or even 32 with the output poweravailable from each channel in the region 50-100 mW. To betterappreciate the advantages of the coiled amplifying devices of FIGS. 41to 47, some detailed examples of how pump light can be injected into thecoil and converted to signal light are presented, and compared to theresults obtainable with a prior-art, double-clad fibre laser.

EXAMPLE IV

This Example is a laser structure formed by coiling a fibre with alongitudinal pump absorption of 50 dB/m at 975 nm. The fibre has anoutside diameter (“OD”) of 50 μm and a core diameter of 10 μm. The coreis single-moded at the signal wavelength and made of Yb³⁺-activatedaluminosilicate glass. The pump absorption cross-section is 20×10⁻²⁵ m²,which implies an Yb³⁺-concentration of 1.44×10²⁶ ions/m³ or about 1.6%by weight. A 10 m long fibre is coiled to a torus of 10 cm diameter,i.e., with approximately 30 turns and with a cross-sectional area ofapproximately 300×300 μm². (Thus, the thickness of the torus is similarto the thickness of a typical double-clad fibre.) The output of threelaser diodes, each at 2 W and with a 100 μm wide stripe are injectedinto the torus with an overall efficiency of 75% via pump couplers madewith 125 μm diameter fibres and equally spaced along the torus. Weestimate that the numerical aperture of the pump beam injected into thetorus is 0.2. The couplers are thus spaced by 10 cm. In order to absorbthe pump, the beam should propagate approximately 20 cm (10 dBabsorption) around the loop, and pass by another pump coupler a singletime. We have estimated the excess loss for light propagating in thecoil upon passing a pump coupler is negligible due to high numericalaperture of the coil and low numerical aperture of the pump opticalfibre. Thus, the design allows essentially the entire pump power to beefficiently converted to signal.

EXAMPLE V

This Example is a laser structure formed by coiling a fibre withlongitudinal pump absorption of 2 dB/m at 975 nm. The fibre has an OD of250 μm and a core diameter of 10 μm. The core is single-moded at thesignal wavelength and made of Yb³⁺-activated aluminosilicate glass. Thepump absorption cross-section is 20×10⁻²⁵ m², which implies anYb³⁺-concentration of 1.44×10²⁶ ions/m³ or about 1.6% by weight. A 200 mlong fibre is coiled to a torus of 10 cm diameter, i.e., withapproximately 600 turns and with a cross-sectional area of about 6×6mm². The output of 10 laser diode sources, each at 20 W and coupled to afibre with 300 μm diameter and with an NA of the beam of 0.2 areinjected into the torus with an overall efficiency of 75% via pumpcouplers which are grouped into pairs and equally spaced along thetorus. It is estimated that the numerical aperture of a pump beaminjected into the torus is 0.3. The couplers are thus spaced by 6 cm. Inorder to absorb the pump, the beam should propagate approximately 5 m(10 dB absorption) around the loop, and in this distance pass by a pumpcoupler 80 times. Since the pump couplers are a small perturbation on athick torus, the excess loss for light propagating in the coil uponpassing a pair of pump couplers will be small, in the region of 0.05 dBor 1%. Thus, with this design, approximately 70% of the pump power willbe usefully absorbed by the Yb³⁺, while the other 30% will be scatteredby the pump couplers.

EXAMPLE VI

This Example is a laser structure formed by coiling a fibre withlongitudinal pump absorption of 0.1 dB/m at 975 nm. The fibre has an ODof 1 mm and a core diameter of 10 μm. The core is single-moded at thesignal wavelength and made of Yb³⁺-activated aluminosilicate glass. Thepump absorption cross-section is 20×10⁻²⁵ m², which impliesYb³⁺-concentration of 1.15×10²⁶ ions/m³ or about 1.3% by weight. A 100 mlong fibre is coiled to a torus of 10 cm diameter, i.e., withapproximately 300 turns and with a cross-sectional area of approximately17×17 mm². The output of 10 laser diode sources, each at 20 W andcoupled to a fibre with 300 μm diameter and with an NA of the beam of0.2 are injected into the torus with an overall efficiency of 75% viapump couplers which are grouped into pairs and equally spaced along thetorus. We estimate that the numerical aperture of a pump beam injectedinto the torus is 0.3. The couplers are thus spaced by 6 cm. In order toabsorb the pump, the beam should propagate approximately 100 m (10 dBabsorption) around the loop, and in this distance pass by a pump coupler1700 times. Because of the very small area of the pump coupler fibrecompared to the torus, we estimate the excess loss for light propagatingin the coil upon passing a pair of pump couplers to 0.1% (0.005 dB).Thus, approximately 55% of the pump power injected into the torus willbe absorbed by the Yb³⁺-ions, and 45% will be scattered by the pumpcouplers.

EXAMPLE VII

This Example is a fibre laser operating at 975 nm. It is well known thatYb ions in silica glass have a large emission cross-section at 975 nmwhich makes a Yb-doped fibre laser a candidate to replace conventionalpigtailed laser diodes operating at this wavelength. Due to three levelnature of the Yb-doped fibre laser at this wavelength the pump powerintensity at the far end of the laser should be in the region of 3·10⁴W/cm² in order to ensure no signal absorption along the laser. Thismeans that for a double clad fibre with a 200 μm outer diameter, thepump-through power will be 10 W, which makes such a laser unpractical.Reducing the fibre OD to 20 μm and transparency power to 100 mW couldmake this laser practical from the required pump power point of view butfibre handling would be extremely difficult. Therefore all previousattempts to realize 975 nm fibre laser based on double clad fibre havehad very limited success. As mentioned above, an advantage of thepresent configuration is that the pump intensity inside the laser can bemade very high provided pump optical fibres are thin enough, which makesa high power 976 nm fibre laser feasible. One possible configuration isbased on 4 W pump diodes operating at 915 nm pigtailed to 200 μm fibre.The fibre is silica rod with silicone rubber cladding. In reasonablyshort length the pump power NA can be kept as low as 0.1 which allowspreservation of pump brightness by tapering output (uncoated) end of thefibre to 20 μm so that pump intensity would be in the region of 10⁶W/cm². 1 m of Yb-doped fibre with pump absorption 10 dB/m at 915 nm iswrapped around a silica tube with 3 cm diameter. The fibre outerdiameter is 120 μm and doped core diameter is 10 μm. The 975 nm laserthreshold is estimated to be in the region of 1.2-1.5 W, slopeefficiency in the region of 80%, and output power in the range of 1-1.5W with one pump diode. Increasing number of pump diodes can scale up theoutput power.

There is given below the following advantages of coiled amplifyingdevices:

-   i. NA=1 suggests OD=65 μm for a 1 kW of pump (actually, NA>1 for an    air-clad fibre);-   ii. Fibre with OD=1 mm is able to handle 4 kW of pump;-   iii. Structure similar to that shown in FIG. 8 can accept virtually    unlimited amount of power (more than 10 kW);-   iv. Insofar as this is an all glass structure, preferably based on    silica glass only, this type of fibre lasers does not suffer from    thermal problems associated with pump absorption and thermally    non-matching materials (glass and silicone rubber, for example);-   v. This type of fibre structure offer better pump absorption due to    larger NA and non-azimuthally symmetrical cross section;-   vi. Pump power can be delivered to the system via dedicated pump    optical fibres with OD=200 μm, NA=0.2 which then can be tapered to    50 μm (still no power loss). The use of modern adhesives can also be    employed;-   vii. The number of pumps is virtually unlimited since by placing    pump optical fibres at different azimuthal positions one can excite    different modes—so there is no pump leakage at entrance points of    adjacent pumps;-   viii. This system offers protection against pump diode failure;-   ix. Pump redundancy capability and flexibility in the range of    output power make this type of fibre amplifier excellent candidate    for leading amplifier for DWDM systems and satellite communications;-   x. Output power obtainable from this type of fibre lasers/amplifiers    can be well beyond 100 W.

EXAMPLE VIII

This Example illustrates the advantages of a parallel optical amplifier580 shown in FIG. 58. The parallel optical amplifier 580 is a preferredembodiment of the parallel amplifier 500 that includes optical isolators581, input fibres 582 and output fibres 583. The Example demonstratesdrastically increased amplification capacity compared to the prior artin a compact, low-cost configuration.

The amplifier 580 has eight independent ports (or amplifying channels)that provide independent amplification, each port comprising the inputfibre 582 connected to the isolator 583, connected to the amplifier 500,connected to another one of the isolators 582 that connects to theoutput fibre 583. Thus, the amplifier 580 can replace eight single-portamplifiers and bring down the amplifier count in a large system bynearly an order of magnitude.

Furthermore, as a result of the abundance of amplifier capacity, theamplifier 580 can be configured in different ways to fulfill differentroles. This Example demonstrates cascading of ports to increase outputpower and bandwidth, as well as independent amplification of eightdifferent wavelength channels.

The Example is based on an optical fibre arrangement that compriseseight Er/Yb co-doped amplifying optical fibres for signal amplificationand two pump optical fibres arranged in such a way that pump powerlaunched into one of the pump optical fibres crosses into all eightamplifying optical fibres via evanescent field coupling.

Each of the amplifier optical fibres 282 has a 100 μm cladding and a 10μm core. The pump optical fibres 281 have a diameter of 125 μm. Fibrescoming out of the amplifier 500 were coated with UV curable secondarycoating. Each end of the amplifying optical fibres 282 was then splicedto a different one of the optical isolators 581 so that the amplifier580 can be considered as a set of eight independent fibre amplifiers.

The pump source 302 was provided by a module comprising six broad stripe915 nm laser diodes coupled into a single 100 μm core, 0.22 NA multimodeoptical fibre. The pump module had built-in laser diode driver andcontrol electronics in a compact package. The pump module can provide upto 8 W of pump power. The pump absorption of the amplifying opticalfibre 282 at this wavelength was approximately 5 dB/m so that the lengthof each amplifier fibre was below 2 m.

The pump optical source 320 was connected to one end of a single pumpoptical fibre 282. In this Example, the unabsorbed pump power wasre-injected into the amplifier 500 by connecting the pump optical fibres281 as shown.

The electrical current for the pump laser diodes was set atapproximately 70% of its maximum value. This protected the amplifier 580against diode failure: one or even two failed diodes can be compensatedfor by a larger pump current to restore the pre-set output power of thesystem.

FIG. 59 depicts the spectral dependence of signal gain for two arbitraryamplifying channels. The gain curves for the other six channels weresimilar. The results demonstrate nearly identical performance of twoindependent amplifiers.

However, the saturation output power from each of the eight amplifiersvaried from 15-18 dBm. The variation is caused by non-uniform pump powerdistribution between individual amplifying optical fibres. Theuniformity can be improved by further developments of the system.

FIG. 60 shows the noise figure. All eight amplifying channels offer a1530-1570 nm gain bandwidth with noise figure below 5 dB. This is veryclose to data for conventional, core-pumped amplifiers. The total outputpower from the amplifier 580 is almost an order of magnitude higher.

The amplifier configuration allows two or more fibre amplifying channelsto be cascaded (as described with reference to FIGS. 34 to 36) in orderto increase the gain or saturated output power, while at the same timeretaining the low noise figure. FIG. 60 shows gain and noise figure forthree cascaded amplifiers. The small signal gain exceeds 50 dB with anoise figure still below 5 dB. The high gain and flexibility of theamplifier assembly allows, for example, for lossy elements likedispersion compensators or switches to be inserted between individualamplifiers, for added functionality without noise or power penalty.

The performance of the amplifying channels was also tested with an arrayof eight distributed feedback DFB fibre lasers with a 50 GHz signalspacing. The DFB fibre lasers were individually pumped and the outputpower was deliberately made unequal with more than 10 dB powervariations. FIG. 61 shows the output power in the individual wavelengthchannels 611, 612, 613, 614, 615, 616, 617, 618. The results demonstratehigh contrast output spectra with significant power equalization.

Another way of using the amplifier 580 is for amplification ofwavelength division multiplexed WDM signals, with the channelsdemultiplexed and then amplified in separate amplifying optical fibres.As seen in FIG. 62, because of the use of a dedicated amplifying opticalfibre for each WDM channel, the amplifier's inter-channel cross-talk isvery low. The cross-talk was measurable only when three amplifiers werecascaded and is below −50 dB.

This Example has demonstrated a zero cross-talk parallel opticalamplifier with small signal gain above 30 dB and noise figure below 5dB. The system comprises eight, parallel, amplifying optical fibrespumped by a compact module with a built-in pump redundancy scheme. Theamplifying optical fibres have a length of 1.5 m, possess low cross-talkand low nonlinear signal distortion. The system can be reconfigured bycascading two or more amplifiers in order to increase gain or saturatedpower, retaining at the same time a very-low noise figure. This parallelamplifier is particularly useful for application inwavelength-division-multiplexed telecommunication networks.

While the above invention has been described in language more or lessspecific as to structural and methodical features, it is to beunderstood, however, that the invention is not limited to the specificfeatures shown and described, since the means herein disclosed comprisepreferred forms of putting the invention into effect. The invention is,therefore, claimed in any of its forms or modifications within theproper scope of the appended claims appropriately interpreted inaccordance with the doctrine of equivalents.

We claim:
 1. An optical amplifier comprising: at least one pump sourceand an optical fibre cable; wherein the optical fibre cable comprises anamplifying optical fibre and a pump optical fibre that are defined byrespective lengths, and the amplifying optical fibre and the pumpoptical fibre are coated with a common coating along a portion of theirrespective lengths; the amplifying optical fibre and the pump opticalfibre are in optical contact with each other along a coating lengthwithin the common coating; the common coating has a refractive indexwhich is lower than a refractive index of a cladding material of thepump optical fibre; the amplifying optical fibre and the pump opticalfibre are made substantially from glass; the amplifying optical fibrecomprises a core and a cladding; the amplifying optical fibre is dopedwith a rare earth dopant; the pump optical fibre is defined by a firstend and a second end; and the first end of the pump optical fibre isconnected to the pump source.
 2. The optical amplifier of claim 1 andfurther comprising at least two pump sources, and wherein the second endof the pump optical fibre is connected to a different one of the pumpsources.
 3. The optical amplifier of claim 1 and comprising a reflectingdevice, and wherein the second end of the pump optical fibre isconnected to the reflecting device.
 4. The optical amplifier of claim 3wherein the reflecting device is positioned proximate the second end ofthe pump optical fibre.
 5. The optical amplifier of claim 1 wherein thepump optical fibre is defined by a first diameter, the amplifyingoptical fibre is defined by a second diameter, and the first diameter isdifferent from the second diameter.
 6. The optical amplifier of claim 1and further comprising an optical feedback arrangement configured topromote light generation within the optical amplifier.
 7. The opticalamplifier of claim 1 and further comprising at least one reflectingdevice configured to reflect optical energy back into the amplifyingoptical fibre.
 8. The optical amplifier of claim 7 wherein thereflecting device is an optical fibre Bragg grating.
 9. The opticalamplifier of claim 7 wherein the reflecting device is a mirror.
 10. Theoptical amplifier of claim 1 wherein the pump optical fibre has anon-circular cross-section.
 11. The optical amplifier of claim 1 whereinthe amplifying optical fibre comprises a lattice of azimuthal holesextending along the axis of the amplifying optical fibre.
 12. Theoptical amplifier of claim 11 wherein the lattice is a regular lattice.13. The optical amplifier of claim 11 wherein the lattice is anirregular lattice.
 14. The optical amplifier of claim 1 wherein the corehas a diameter in the range of 2 μm to 100 μm.
 15. The optical amplifierof claim 1 wherein the optical amplifier provides optical amplificationof an input signal, and the optical amplification is providedsubstantially by the coating length in which the pump optical fibre andthe amplifying optical fibre are coated and are in optical contact witheach other.
 16. A laser comprising the optical amplifier of claim
 6. 17.A laser comprising the optical amplifier of claim 7.